One application for robust generation of an index pulse is needed in an electric power steering system (EPS). EPS has been the subject of development by auto manufacturers and suppliers for over a decade because of its fuel economy and ease-of-control advantages compared with the traditional hydraulic power steering (HPS). However, commercialization of EPS systems has been slow and is presently limited to small and midget-class cars due to cost and performance challenges. Among the most challenging technical issues is the pulsating feel at the steering wheel and the audible noise associated with the type of high performance electric drives needed to meet the steering requirements.
The choice of motor type for an EPS is a crucial one, since it determines the characteristics of the drive and the requirements on the power switching devices, controls, and consequently cost. Leading contenders are the Permanent Magnet (PM) brushless motor, the Permanent Magnet (PM) commutator-type and the switched reluctance (SR) motors, each of the three options has its own inherent advantages and limitations. The PM brushless motor, was chosen based on years of experimenting with commutator-type motors. The large motor size and rotor inertia of commutator-type motors limit their applicability to very small cars with reduced steering assist requirements. Additionally, the potential for brush breakage that may result in a rotor lock necessitates the use of a clutch to disconnect the motor from the drive shaft in case of brush failure. SR drives offer an attractive, robust and low cost option, but suffer from inherent excessive torque pulsation and audible noise, unless special measures are taken to reduce such effects. For column assist applications, the motor is located within the passenger compartment and therefore must meet stringent packaging and audible noise requirements that the present SR motor technology may not satisfy. Therefore, the PM brushless motor with its superior characteristics of low inertia, high efficiency and torque density, compared to commutator motors, appears to have the potential for not only meeting the present requirements but also of future high performance EPS systems of medium and large vehicles.
Despite the relatively low levels of torque ripple and noise of EPS systems using conventional PM brushless motors, they are no match to the smoothness and quietness of HPS with decades-long history of performance refinement efforts. Consumers are reluctant in compromising such features. Therefore, a new torque ripple free (TRF) system is needed, which as the name indicates would eradicate the sources of torque ripple (under ideal conditions) and reduces the noise level considerably. The near term goal is to enhance the performance of EPS systems with the long term objective of increasing acceptability of EPS systems for broader usage.
Several performance and cost issues have stood in the way of broad-based EPS commercialization regardless of the technology used, but with varying degree of difficulty. This requires that the following be addressed:
Smooth steering feel of the TRF system can still be affected by transients in assist torque provided by the motor drive due to limitations in motor design and controller operation. For example, the cogging torque due to magnetic interaction between the permanent magnets and the slotted structure of the armature can induce a torque ripple. Another torque ripple at the motor electric frequency can be generated if the current supplied to the motor has a small amount of DC offset due to an inaccuracy in current sensor feedback or other controller related errors. Torque ripples as described above may be understood as a consistent variation of torque level taking place at specific times or specific locations of the rotor. A different source of torque transient is an unexpected or faulty change in torque magnitude or direction. It may be worse inasmuch as it may happen at any time, and may be small or large. Such faulty change in torque must be avoided, and the present invention teaches how to eliminate one possible source of such faulty torque.
Typically, the TRF-EPS system requires a position encoder located on the motor shaft. This encoder provides an incremental, high resolution pulse train; three lower resolution signals used for motor commutation, i.e. to direct the sinusoidal currents to the proper phase; and, an index pulse for the proper timing of the sinusoidal currents. The index pulse is usually derived from the low-resolution commutation signals. Small timing variations occur in the index pulse generating circuit due to hysteresis, temperature changes, aging, etc. Such small timing variations can produce faulty index pulses. A specific class of faulty index pulses were found to occur during changes of steering direction. When a faulty index pulses occurs, an undesirable torque transient follows. It may result in a strong kick at the steering wheel.